Bearings are used in a variety of machines and structures. A bearing is generally configured to enable relative movement of two different components or structures between which the bearing is positioned. In one example, one or more bearings may be configured to enable substantially continual displacement of such components, such as rotors and stators in a motor or pump design. In another example, bearings are used, as “expansion” or “isolation” type bearings in association with a large structure such as a road, a bridge or a building.
Considering expansion and isolation type bearings, such bearings are used to accommodate the inherent expansion and contractions that occur in structures such as roads and bridges. There are various difficulties in designing such bearings. For example, the expansion and contraction of roads and bridges (and other structures) often occur due to continually changing temperature conditions. Additionally, cycling loads (e.g., from vehicles passing over a bridge) or occasional forces of nature (e.g., earthquakes and winds) cause movements in large, man made structures. Thus, considering bridges as example, engineers must be able to design bridge joints with bearings that can both accommodate extreme loads (e.g., the bridge structure as well as the vehicles that use the bridge) while also enabling relative displacement of mating components without significant forces and stresses being generated within the structure since excessive forces can easily damage the structure requiring significant costs to repair if not catastrophic failure. Another issue associated with heavy load bearings is the ability to provide a bearing that resists corrosion from the elements such as moisture and salt. Additionally, a heavy load bearing desirably exhibits a long service life and requires relatively little maintenance.
Most conventional bridge bearings are currently made of a polymer material such as polytetrafluoroethylene (PTFE) or an elastomer material. Often, such as in the case of an elastomer bearing, the polymer materials are produced in sheets and designed to move in shear. In other words, the sheet of polymer material becomes strained upon relative displacement of associated bridge components such that the upper surface of the sheet of polymer material is displaced some distance relative to the lower surface of the sheet. For example, referring briefly to FIGS. 1A and 1B, a prior art elastomer bearing 20 is shown including a lower plate 22, an upper plate 24 and an elastomer sheet 26 disposed between the two plates 22 and 24. The lower plate 22 is coupled to a foundational structure 28 and the upper plate is coupled to a structural component 30 of, for example, a bridge or a building. FIG. 1A shows the bearing 20 in a stable or “at rest” condition. FIG. 1B shows the bearing after the structural component 30 has been displaced relative to the foundational structure 28 as indicated by directional arrow 32. This displacement causes the elastomer sheet 26 to distort (as indicated by the angled end surfaces of the elastomer sheet 26 shown in FIG. 1B).
In some bearings, the polymer material may be laminated such that multiple sheets of polymer material are used, sometimes with a structural reinforcing member (e.g., a metal sheet) disposed between each adjacent pair of polymer sheets. However, there are various limitations associated with such bearings that employ polymer sheets.
For example, when used in heavy load applications, elastomers can be limited in the lateral movement that they can accommodate for the loads they must bear. In many applications, horizontal movement can easily be many inches in multiple directions which is not easily tolerated by a thicker elastomer bearing (as may be required due to expected loadings), even when laminated in construction. Additionally, elastomers are conventionally susceptible to material property changes over time. Elastomer bearings may also be prone to “walk” out of their position between adjacent bearing plates over time. Environmental conditions are also a concern for elastomer bearings. For example, elastomer materials often experience deterioration due to exposure to ozone. Material creep can also be an issue when using elastomer bearings.
Referring briefly to FIGS. 2A and 2B, another prior bearing 40 is shown. The bearing 40 is configured as a PTFE sliding bearing and includes a lower plate 42, a layer of PTFE 44 bonded to the lower plate, and an upper plate 46 configured to slide across the layer of PTFE 44. The lower plate 42 is coupled to a foundational structure 48 and the upper plate is coupled to a structural component 50 of, for example, a bridge or a building. FIG. 2A shows the bearing 20 in a first position and FIG. 1B shows the bearing after the structural component 50 has been displaced relative to the foundational structure 48 as indicated by directional arrow 52. Ideally, the PTFE layer 44 is not distorted (as with the elastomer bearing), but rather accommodates mutual displacement of the two plates 42 and 46 due to the low coefficient of friction of the PTFE material. As with elastomer type bearings, there are drawbacks in using PTFE sliding bearings.
In bearing applications, PTFE has a limited service life, even under ideal conditions. However, ideal conditions are not the norm with respect to bridges and other heavy load applications. Rather, such applications often provide harsh and dirty environments for the bearing, elevating the risk of failure in PTFE. As such, added care is required in designing and maintaining PTFE bearings in an effort to keep them clean.
Some reports indicate that some PTFE failures are due to uneven or excessive loading in localized areas of the bearing surface. Additionally, PTFE is not a preferred material when the bearing is anticipated to experience relatively fast movements. Further, PTFE is susceptible to creep or cold flow under higher compressive loads and is generally prone to wear as movement occurs and it will require replacement over time. All of these issues make PTFE a material that is susceptible to failure in conditions experienced by slide bearings that may be used, for example, in a bridge or other heavy load application.
Other bearings are used in heavy load applications such as bronze sliding plate bearings, metal plate bearings having a graphite-impregnated asbestos sheet between the metal bearing plates, rocker bearings, roller bearings and pin-and-link bearings. However, all of these bearings exhibit shortcomings and are prone to wear, corrosion and deterioration due to the service demands placed on the bearings in relatively harsh environments. For example, in many of these bearings, including sliding plate bearings where a metal plate is used to provide the bearing surface, such bearings may freeze such that the bearing no longer slides or rotates as originally designed. The freezing of a bearing may occur due to a variety of reasons including corrosion, mechanical binding, dirt buildup or wear of one bearing component by the mating bearing component (e.g., due to localizing application of forces).
It is a continual desire in the industry to develop bearings that provide benefits in one or more categories such as mechanical strength, corrosion resistance, wear resistance, extended service life, and low coefficient of friction.